In preliminary experiments, we were able to demonstrate that the administration of methionine can inhibit the progression and cause a remission of hepatocellular carcinoma induced by DL-ethionine in rats. For this reason it is important to establish the basic conditions for methionine action and its specificity and to make a preliminary approach to the elucidation of its mechanism. The possibility of intervention in the progression of malignancy in vivo is a very attractive idea, the success of which would have profound theoretical impact and far reaching clinical consequences.